Hot melt adhesives typically exist as a solid mass at ambient temperature and can be converted to a flowable liquid by the application of heat. These adhesives are particularly useful in manufacturing a variety of disposable goods where bonding of various substrates is often necessary. Specific applications include disposable diapers, hospital pads, feminine sanitary napkins, panty shields, surgical drapes and adult incontinent briefs, collectively known as disposable nonwoven hygienic products. Other diversified applications have involved paper products, packaging materials, automotive headliners, appliances, tapes and labels. In most of these applications, the hot melt adhesive is heated to its molten state and then applied to a substrate, often named as the primary substrate. A second substrate, often named as the secondary substrate, is then immediately brought into contact with and compressed against the first. The adhesive solidifies on cooling to form a strong bond. The major advantage of hot melt adhesives is the absence of a liquid carrier, as would be the case of water or solvent based adhesives, thereby eliminating the costly process associated with solvent removal.
For many applications, hot melt adhesives are often extruded directly onto a substrate in the form of a thin film or a bead by using piston or gear pump equipment. In this case, the substrate is brought into intimate contact with a hot die under pressure. The temperature of the die must be maintained well above the melting point of the adhesive to allow the molten hot melt material to flow through the application nozzle smoothly. For most applications, particularly those encountered in food packaging and disposable nonwovens hygienic article manufacturing, bonding of delicate and heat sensitive substrates, such as thin gauge plastic films, is often involved. This imposes an upper limit on coating temperature for hot melt adhesive applications. Today's commercial hot melts are typically formulated to have coating temperature below 200° C., preferably below 150° C., to avoid substrate burning or distortion. Besides directly coating, several indirect or noncontact coating methods, through which a hot melt adhesive can be spray coated with the aid of compressed air onto a substrate from a distance, are also developed. These non-contact coating techniques include conventional spiral spray, Omega™, Surewrap™ and various forms of melt-blown methods. The indirect method, however, requires that the viscosity of the adhesives must be sufficiently low, usually in the range of 2,000 to 30,000 mPa·s, preferably in the range of 2,000 to 15,000 mPa·s, at the application temperature in order to obtain an acceptable coating pattern. Many other physical factors, especially the rheological properties of the adhesive, come into play in determining the sprayability of a hot melt. The majority of commercial hot melt products do not lend themselves to spray applications. There are no accepted theoretical models or guidelines to predict sprayability, which must be determined empirically with application equipment.
Hot melt adhesives are organic materials typically consisting of a polymer, a plasticizer, a tackifying resin, and an antioxidant package. Other ingredients, such as wax, filler, colorant and UV absorber, can also be used to modify the adhesive properties or to provide special attributes. These organic ingredients are prone to heat degradation under the coating conditions of the adhesive. For example, the widely used commercial hot melt adhesive based on styrene-isoprene-styrene (SIS) triblock copolymer, when subjected to 175° C. for 24 hours, can suffer from a viscosity drop of about 50 percent from its original value. A styrene-butadiene-styrene (SBS) based hot melt may cause problems by crosslinking under similar conditions. Crosslinking can result in a dramatic increase in viscosity and may eventually render the adhesive un-flowable by the formation of three dimensional polymer network. The viscosity change is often accompanied by charring, gelling, and formation of skin on top of the molten material. The degradation will inevitably lead to deterioration of the adhesive properties and performance. In addition, they can also cause equipment damage. The rate of degradation is temperature dependent; the higher the temperature, the faster the degradation. Thus, reducing the coating temperature of the adhesive can slow down degradation.
Conventional polyolefins produced by using Ziegler-Natta (ZN) catalysts such as, for example, low density polyethylene (LDPE), high density polyethylene (HDPE), and istotatic polypropylene (iPP) do not lend themselves to adhesive applications. Ziegler-Natta (ZN) catalyst systems consist of a pair of catalyst and co-catalyst. The most common of such pairs are TiCl3 and Al(C2H5)2Cl, or TiCl4 with Al(C2H5)3. Ziegler-Natta catalyst systems are a subject of numerous publications in scientific journals and textbooks and are well known to those skilled in the art. A conventional ZN catalyst system is typically embedded in an inert support and has several catalyst sites, each of which has different activity. This difference in activity causes the formation of polymer molecules with a plurality of molecular weights and composition of copolymer molecules. The polyolefin homopolymers and copolymers produced with ZN catalysts are typically highly crystalline and stiff. This can translate to a hot melt adhesive that is relatively brittle or has poor substrate wetting, poor adhesion, and poor processibility. Nevertheless, hot melt adhesives containing various types of polyolefin blends are known in various previous patent literatures.
As used herein, Z-N refers to a Ziegler-Natta catalyst for olefin polymerization.
As used herein, LDPE and HPDE refer to low density polyethylene and high density polyethylene, respectively.
As used herein, iPP refers to isotactic propylene homopolymers or copolymers having predominantly an isotactic chain structure.
As used herein, amporphous poly-alpha-olefin (APAO) refers to a class of low molecular weight amorphous propylene homopolymers or copolymers with ethylene or butene typically produced with a Lewis acid catalyst.
As used herein, PB refers to polybutene homopolymers and copolymers.
For example, Trotter et al, in U.S. Pat. No. 4,022,728, describes a hot melt pressure sensitive composition comprising a mixture of APAOs, a low molecular weight substantially amorphous elastomer, a liquid tackifier, and a conventional crystalline polypropylene (iPP) in the amount of up to 2% by weight. According to the '728 patent, the composition provides good adhesive properties at low temperatures.
Meyer et al, in U.S. Pat. No. 4,120,916, discloses hot melt adhesive compositions comprising a blend of low molecular weight PE, low molecular weight iPP, and APAO. These adhesive compositions are said to offer short open times and to be useful for bonding of paraffin modified corrugated board.
Lakshmanan et al, in U.S. Pat. No. 4,761,450, discloses a polymer blend useful as a hot melt adhesive comprising a LDPE, a copolymer of butene-1 with ethylene or propylene, a hydrocarbon tackifier, and a low molecular weight polymer consisting of a low molecular weight liquid polybutene, an APAO, and mixtures thereof.
Ryan discloses in U.S. Pat. No. 5,747,573 an APAO based hot melt adhesive composition useful for bonding plastics and metallized foil containers. The adhesive composition contains a blend of APAO, a solid benzoate plasticizer, and a hydrocarbon tackifier.
Blending APAO with polyethylene (PE), polybutene (PB) copolymers, or the conventional iPP leads to severe drawbacks. The prior art adhesives containing APAO/PE or APAO/PB blends, such as, for example, those described herein above in U.S. Pat. Nos. 4,120,916 and 4,761,450 tend to have poor compatibility. These adhesives can undergo phase separation during the application process when the hot melt adhesive must be kept in the molten state at high temperature for a prolonged period of time, sometimes for hours or even days. Charring, skinning and gelling can develop rather quickly in the phase separated hot melt adhesives, thereby causing the application equipment to block or plug-up. The incompatibility of such polymer blends also imparts brittleness, optical haziness, poor or no open time, and low bond strength. Although APAO and the conventional iPP blend based hot melts do not have the compatibility problems, they may still suffer from all the other drawbacks herein described above. Moreover, due to high crystallinity and high melting point of the conventional iPP polymers, hot melt adhesives based on APAO/iPP blends tend to be hard and brittle unless the iPP polymer amount is kept at a very low level, such as, for example, at about or below 2% by weight as disclosed in U.S. Pat. No. 4,022,728. As a result, these adhesives will have poor tensile strength, poor bond strength, and poor impact resistance. Another detrimental effect of iPP is the increased coating temperature. The adhesive must be heated well above the melting point of iPP (ranging from 180 to 200° C.) to reach liquid state. Although the blend of high and low molecular weight atactic polyolefin approach described in U.S. Pat. No. 5,723,546 offers some improvement on tensile properties of APAO, it has not been able to provide sufficient tensile strength and high temperature properties to overcome the deficiencies of sole APAO based hot melts.
The shortcomings in the prior art mentioned above are partially overcome in more recent inventions that are disclosed in U.S. Pat. No. 6,329,468, that teaches the use of semicrystalline flexible polyolefin for hot melt adhesive compositions; in U.S. Pat. No. 7,262,251 that teaches a hot melt adhesive composition based on a random copolymer (RCP) of isotactic polypropylene and a secondary polymer; in US Patent Application Publication US2003/0096896 A1 that describes a hot melt composition comprising a blend of syndiotactic polypropylene (sPP) and APAO; in U.S. Pat. No. 8,383,731 that describes an adhesive blend based on a semicrystalline copolymer of propylene with an alpha-olefin. These compositions, however, all consist of a rigid semicrystalline polymer that is non-uniform in either intramolecular and/or intermolecular compositional distribution, and in the tacticity distribution of molecular chains. It is not the intention of the present invention to dwell on theoretical discussions of polymer property-function relationships, but non-uniformity in the composition and chain structure, coupled with a very broad molecular weight distribution, may be accountable for poor adhesive properties and poor processibility for a hot melt composition. These semicrystalline polymers in the above compositions can have rigid polymer chain structure, which is detrimental to adhesion and application properties of hot melt adhesives containing such polymers. It is extremely difficult, if not impossible, to balance the complex requirements of adhesion, cohesion, low viscosity, broad application temperature range, and applicability by a whole array of application methods.
Most recently, Tse et al in U.S. Pat. No. 9,109,143 revealed an adhesive composition containing a blend of two low molecular weight propylene based copolymers having a weight average molecular weight (Mw) less than 100,000 g/mol. The low molecular weight propylene copolymers of the '143 patent also have low melting point and low crystallinity. The copolymers, primarily directed to sealing of corrugated boxes, have poor cohesive strength and therefore do not lend themselves to an array of demanding applications such as, for example, in elastic attachment for nonwoven hygiene products and auto headliner assembly.
U.S. Patent Application No. 2016/0121014 discloses a disposable absorbent article and an adhesive composition including a first polymer that is propylene-based and has a molecular weight of no greater than about 75,000 and a second polymer selected from a group including propylene based polymers with a molecular weight of at least about 100,000 and styrene block copolymers with a styrene content of no greater than about 20%, where the adhesive composition is alleged to be useful for elastic attachment applications.